This invention relates to casting processes using investment molds as used in the lost wax casting techniques. The term, lost wax casting, also applies to the use of plastic materials for making patterns.
The basic steps in making an investment casting is the casting of the pattern in a non-wetting, air porous investment material which is contained in a metal flask. The flask containing the pattern in the investment is then baked at temperatures high enough to reduce the pattern to gas and ashes which are expelled through a hole in the top on the flask. While the flask is still at temperature, molten metal is introduced into the hole in the flask.
The casting art is very old having been used thousands of years ago but like any other art it has been subject to continual improvements. For large casting, gravity is sufficient to cause the metal to flow into the mold but for small delicate casting, gravity has been implemented by applying pressure to the molten metal or by using a vacuum to pull the metal into the mold or by a combination of both pressure and vacuum. The increase in costs of precious metals in the jewelry trade has introduced the challenge of finer and smaller castings.
In fine castings the surface-to-volume ratio is high and thus heat is rapidly being extracted from the molten metal through the relatively large surface while it is flowing into the crevices of an intricate casting. Freezing of the molten metal can and does occur before the molten metal has completely filled the mold. Contributing to the freezing of the metal before the mold is completely filled is compounded by the surface tension effect and the remnant air in the mold cavity. Thus heat is rapidly being extracted from the molten metal. The surface tension effect is the tendency for liquid metal droplets to "ball up" when sitting on a non-wetted surface. These problems in investment casting can be solved by maintaining the superheat of the molten metal until the mold cavity is completely filled and pressure and/or vacuum is applied to force the melt into the crevices.
In the present state-of-the-art pressure is applied by a piston, steam, air or the centripetal force of a centrifuge. The most widely used method is the centrifuge, usually energized by a spring motor which accelerates the flask to a rotary speed in the vicinity of 5 to 8 revolutions per second with a maximum pressure in the mold by the metal of 10 to 20 pounds per square inch. The pressure starting at zero then increasing to maximum pressure as the centrifuge accelerates to maximum rotational speed then reducing to zero as the centrifuge coasts to a stop. This pressure is not constant in the mold as the radius to the center of rotation varies as the length of the mold. There is a certain amount of injury risk in the operation of the centrifuge. Problems of pressure control and the rate of application of pressure are similarly inherent in the steam, vacuum or piston systems as observed in the present state-of-the-art.
The following U.S. Patents and/or other prior art disclose such patents.
U.S. Pat. Nos. 1,758,380, 2,369,277, 1,962,456, 2,785,448, 3,123,875, 3,705,615, 3,712,364, 3,903,953, 4,245,690.